Satellites in orbit experience a number of factors such as aerodynamic drag that can cause undesirable changes in attitude. To maintain proper attitude, various mechanism such as control moment gyroscopes and reaction wheel assemblies are included in the satellite and are used to periodically adjust the attitude of the satellite. These systems also allow the vehicle to be rotated so as to point towards other objects in space. For example, a satellite containing a telescope could be rotated to point at a selected star or planet. These control systems are generally employed in circumstances where longevity and high accuracy over extended periods of time are of paramount importance. The performance of these devices is largely determined by the predictability of the behavior of the gyro rotor. This behavior, in turn, is influenced by the geometry, friction, compliances, torque, environmental considerations and the operating characteristics of the bearings that support the rotor.
Traditionally, gyro rotors are supported by ball bearings. The ball bearing art is well advanced and excellent results using ball bearings are obtainable. However, the various relatively unpredictable characteristics occasioned by the presence of balls, raceways, and the physical interaction between moving surfaces can have various undesired effects on the performance of the bearings which effects, at best, can be minimized but not eliminated.
One fairly common arrangement provides a rotor that is supported for rotation about its spin axis by a pair of oppositely disposed hubs that are each journaled in a pair of spin bearings. Typically, one of the bearing pairs may be a floating bearing assembly that includes a rotatable member or shaft coupled to the rotor shell and which is rotatable about a spin axis. The bearing assembly further incorporates a floating bearing cartridge which includes a pair of duplex bearings having inner and outer bearing races and a common outer sleeve assembly which serves to clamp the duplex bearing outer races under a predetermined preload. The outer surface of the bearing cartridge is permitted to translate along the spin axis under changes in ambient operating temperature or pressure differential.
The floating bearing cartridges are typically cylindrical and are contained in a cylindrical housing that provides clearance between the bearing cartridge and the cartridge sleeve assembly. This annulus is normally filled with a viscous fluid to promote heat transfer from the bearing cartridge and to also provide viscous shear damping for axial vibrations. These undesirable axial vibrations are typically caused by the axial motions generated by imperfections in the bearing and shaft geometries.
While various types of viscous fluids have been employed to increase the performance of the bearings with respect to reducing axial vibrations; pressure, temperature and other operating conditions associated with the environment of the gyroscope can negatively impact the performance of the viscous fluids used to damp axial vibrations. Additionally, the desire to significantly increase the rate of rotation for the gyro from the present rate of approximately 6,000 RPM has presented new performance considerations in certain applications. Specifically, as the rate of rotation for the rotor is increased to the range of 30,000–40,000 RPM, the radial and axial vibrations associated with the spinning rotor can be significantly magnified. When operating at a speed associated with a structural resonance, these rotor-generated forces can become significant enough to potentially cause structural damage.
These various factors have made it difficult to uniformly and predictably select a fluid with the appropriate viscosity to provide the desired results in the area of axial damping for control moment gyroscopes, reaction wheel assemblies, and the like. In fact, if the incorrect viscous fluid is chosen, it may actually provide viscous coupling between the rotor and the housing instead of viscous damping for these axial vibrations. Once again, this viscous coupling is very undesirable and may cause structural damage to the assembly.
In view of the foregoing, it should be appreciated that it would be desirable to provide methods for improving the selection of the viscous fluid used in the bearing annulus of control moment gyroscopes, reaction wheels, and momentum wheels, thereby enhancing the damping of the undesirable axial vibrations, specifically at high rates of rotation.